There is no direct way to analyze the polymeric structures of melamine-urea-formaldehyde (MUF) resins because of the lack of means to determine the amounts of the bonds which might be formed between melamine and urea residues. Therefore, deduction from the curing behavior of the resin is indispensable to obtain some information on the polymeric structure of the MUF resin. One way of describing the curing behavior of a thermosetting resin is to express the quantitative and qualitative-changes of the three-dimensionally cross-linked fraction (gel fraction) as a function of time. Thus the methods for separating the gel fraction from the sol fraction and for analyzing the composition of the gel fraction were investigated. A two-step extraction technique with an aqueous 4% sodium hydroxide solution and acetic acid was effective for the separation of the gel fractions of the MUF resins being cured. In order to analyze the composition of the gel fraction from its nitrogen content theoretical calculations were worked out on the relationships between the degrees of conversions and the nitrogen contents of the gel fractions assuming that the conversion of the urea part of the resin into gel occurred after completion of the conversion of the melamine part. Comparison of the theoretical values with experimental ones revealed that the conversion of the melamine part preceded that of the urea part, although the conversion of the urea part took place before that of the melamine part was completed. The analysis provided an estimate for the urea/melamine (U/M) molar ratio of the gel fraction. In an early stage of the curing course of the MUF resins with U/M molar ratios of 1-1.5 the gel fractions were estimated to be composed of 1 mole of urea to 2-2.5 moles of melamine. The presence of the urea residues in the gel fraction at this stage of curing was attributable to the co-condensation between urea and melamine from a comparison of the rates of gel formation of urea-formaldehyde resins with those of melamine-formaldehyde resins.
